sms: SMS objects need to be create by `create_modem()' in the Messaging iface

So that plugins can subclass the generic SMS object.

src/mm-iface-modem-messaging.c

@@ -39,6 +39,16 @@ mm_iface_modem_messaging_bind_simple_status (MMIfaceModemMessaging *self,
 
 /*****************************************************************************/
 
+MMSms *
+mm_iface_modem_messaging_create_sms (MMIfaceModemMessaging *self)
+{
+    g_assert (MM_IFACE_MODEM_MESSAGING (self)->create_sms != NULL);
+
+    return MM_IFACE_MODEM_MESSAGING_GET_INTERFACE (self)->create_sms (self);
+}
+
+/*****************************************************************************/
+
 typedef struct {
     GArray *supported_mem1;
     GArray *supported_mem2;